Zero Suit is probably one of the worst ledge guarders for ROB, solely because of her range. ROB doesn't have a lot of ledge options (ledge attack, drop down-> jump gyro/ fair/ uair, or jumping over her). Zero suit's fast up B, long range up smash, and quick uair make jumping above her very difficult, and she'll often be out of range of his ledge attack. And losing ledge invincibility is quite risky/ less predictable.
The other high tiers don't have the disjoints that she has, thus it certainly gives her the edge in that respect. For ROB- on most of the high tier, a fair is a pretty solid option, but it's much tougher to get close to her, opposed to the other high tiers.
I'd rather eat a hit from Zero Suit's up air, rather than Rosalina/ Diddy/ Yoshi/ Sheik/ fox. At least from a Zero Suit, you probably won't die.
As far as juggling goes, ROB doesn't fall at the same speed that Zero Suit does. I've found her frame traps to be annoying, but she often has to use her second jump if he is slightly out of reach. It depends on the precision of the zero suit/ if ROB has his jump/ etc. If she is positioned correctly, ROB takes a hit, and be sent out of reach of another hit. So it's likely that he'll only take one hit before she lands on the ground. After which, I'm either charging up a fast fall nair/ falling to the other side of the stage/ tossing gyros where Zero Suit might land/ etc. That's opposed to characters, like Diddy or Luigi, who can get ~2-3 up airs hits off in one jump. She's not nearly as scary as some of the other top dogs' up airs.